This application claims the benefit of United Kingdom Patent Application No. GB0803517.2, filed on Feb. 27, 2008, which hereby is incorporated by reference in its entirety.
In the field of oil and gas exploration and production, various tools are used to provide a fluid seal between two components in a wellbore. Isolation tools have been designed for sealing an annulus between two downhole components to prevent undesirable flow of wellbore fluids in the annulus. For example, a packer may be formed on the outer surface of a completion string which is run into an outer casing or an uncased hole. The packer is run with the string to a downhole location, and is inflated or expanded into contact with the inner surface of the outer casing or openhole to create a seal in the annulus. To provide an effective seal, fluid must be prevented from passing through the space or micro-annulus between the packer and the completion, as well as between the packer and the outer casing or openhole.
Isolation tools are not exclusively run on completion strings. For example, in some applications they form a seal between a mandrel which forms part of a specialised tool and an outer surface. In other applications they may be run on coiled tubing, wireline and slickline tools.
Conventional packers are actuated by mechanical or hydraulic systems. More recently, packers have been developed which include a mantle of swellable elastomeric material formed around a tubular body. The swellable elastomer is selected to expand on exposure to at least one predetermined fluid, which may be a hydrocarbon fluid or an aqueous fluid. The packer may be run to a downhole location in its unexpanded state, where it is exposed to a wellbore fluid and caused to expand. The design, dimensions, and swelling characteristics are selected such that the swellable mantle expands to create a fluid seal in the annulus, thereby isolating one wellbore section from another. Swellable packers have several advantages over conventional packers, including passive actuation, simplicity of construction, and robustness in long term isolation applications. Examples of swellable packers are described in GB 2411918.
FIG. 1 of the drawings shows a swellable packer according to the prior art, generally depicted at 10, formed on a tubular body 12 having a longitudinal axis L. The packer 10 comprises an expanding mantle 14 of cylindrical form located around the body 12. The expanding mantle 14 is formed from a material selected to expand on exposure to at least one predetermined fluid. Such materials are known in the art, for example from GB 2411918.
As illustrated in FIGS. 2A and 2B, the dimensions of the packer 10 and the characteristics of the swellable material of the expanding portion 14 are selected such that the expanding portion forms a seal in use, which substantially prevents the flow of fluids past the body 12. FIG. 2A is a cross-section through the packer 10 located in a wellbore 20 in a formation 22. On exposure to a wellbore fluid in the annulus 24, in this case a hydrocarbon fluid, the expanding portion 14 expands and its outer diameter increases until it contacts the surface 26 of the wellbore to create a seal in the annulus 24. The seal prevents flow of fluid in the wellbore annulus between a volume above the packer 10 and a volume below the packer 10. Although shown here in use in an uncased hole, the packer 10 could of course be used in a cased hole, in which case the mantle would form a seal against the interior surface of the outer casing.
Typically a packer will be constructed for a specific application and incorporated into a casing string or other tool string by means of threaded couplings. Swellable packers are typically constructed from multiple layers of uncured elastomeric material, such as EPDM. Multiple layers are overlaid on a mandrel or tubular in an uncured form to build up a mantle of the required dimensions. The mantle is subsequently cured, e.g. by heat curing or air curing. The outer surface of the swellable mantle is then machined using a lathe to create a smooth cylindrical surface. This method produces a fully cured, unitary swellable mantle capable of sealing large differential pressures. However, the process is generally labour-intensive and time consuming, and the uncured material can be difficult to handle. Moreover, the resulting expanding portion, although robust and capable of withstanding high pressures, may be ill-suited to some downhole applications.
In wellbore construction, cement is used to seal an annulus between a casing section and an openhole, or an annulus between two concentric tubulars, to prevent undesirable fluid flow to surface. Large volumes of cement are required to seal an annulus from a casing point back to surface, and when the casing is cemented into place, the cement forms a structural component of the wellbore.
There is generally a need to provide sealing mechanisms and isolation tools and systems which may be manufactured and assembled more efficiently than in the case of the prior art, and which are flexible in their application to a variety of wellbore scenarios.